272 research outputs found
Accelerating charging dynamics in sub-nanometer pores
Having smaller energy density than batteries, supercapacitors have
exceptional power density and cyclability. Their energy density can be
increased using ionic liquids and electrodes with sub-nanometer pores, but this
tends to reduce their power density and compromise the key advantage of
supercapacitors. To help address this issue through material optimization, here
we unravel the mechanisms of charging sub-nanometer pores with ionic liquids
using molecular simulations, navigated by a phenomenological model. We show
that charging of ionophilic pores is a diffusive process, often accompanied by
overfilling followed by de-filling. In sharp contrast to conventional
expectations, charging is fast because ion diffusion during charging can be an
order of magnitude faster than in bulk, and charging itself is accelerated by
the onset of collective modes. Further acceleration can be achieved using
ionophobic pores by eliminating overfilling/de-filling and thus leading to
charging behavior qualitatively different from that in conventional, ionophilic
pores
Features of the unconventional gas deposits development
This article generalizes the data on reserves and production of shale gas in different countries. It also analyzes the peculiarities of the geological structure and development of the shale gas and gas in dense rock deposits. As to the development of shale gas deposits with a net of horizontal wells with multi-stage hydraulic bed fracture, it presents the effect of the factor of the rock matrix permeability, half-length fracture and the distance between fractures on the gas recovery
Critical mass and the dependency of research quality on group size
Academic research groups are treated as complex systems and their cooperative
behaviour is analysed from a mathematical and statistical viewpoint. Contrary
to the naive expectation that the quality of a research group is simply given
by the mean calibre of its individual scientists, we show that intra-group
interactions play a dominant role. Our model manifests phenomena akin to phase
transitions which are brought about by these interactions, and which facilitate
the quantification of the notion of critical mass for research groups. We
present these critical masses for many academic areas. A consequence of our
analysis is that overall research performance of a given discipline is improved
by supporting medium-sized groups over large ones, while small groups must
strive to achieve critical mass.Comment: 16 pages, 6 figures consisting of 16 panels. Presentation and
reference list improved for version
Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study
We present zero field and transverse field \muSR experiments on the recently
discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero
field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples
rule out any static magnetic order above 1.6 K in these superconducting
samples. From transverse field experiments in the vortex phase we deduce the
temperature and field dependence of the superfluid density. Whereas the
temperature dependence is consistent with a weak coupling BCS s-wave or a dirty
d-wave gap function scenario, the field dependence strongly evidences
unconventional superconductivity. We obtain the in-plane penetration depth of
\lambda_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and \lambda_{ab} (0) =
364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventional
superconductivity is provided by the ratio of T_c versus the superfluid
density, which is close to the Uemura line of hole doped high-T_c cuprates.Comment: 5 pages, 5 figure
Optical Study of LaO_0.9F_0.1FeAs: Evidence for a Weakly Coupled Superconducting State
We have studied the reflectance of the recently discovered superconductor
LaO_0.9F0.FeAs in a wide energy range from the far infrared to the visible
regime. We report on the observation of infrared active phonons, the plasma
edge (PE) and possible interband transitions. On the basis of this data and the
reported in-plane penetration depth lambda_L(0) about 254 nm [H. Luetkens et
al., Phys. Rev. Lett. v. 101, 0970009 (2008)] a disorder sensitive relatively
small value of the total electron electron-boson coupling constant
lambda_tot=lambda_e-ph+lambda_e-sp ~ 0.6 +- 0.35 can be estimated adopting an
effective single-band picture.Comment: Changed title, updated references, final published versio
High-Field Pauli-Limiting Behavior and Strongly Enhanced Upper Critical Magnetic Fields near the Transition Temperature of an Arsenic-Deficient LaO_0.9F_0.1FeAs_(1-\delta) Superconductor
We report upper critical field B_c2(T) data for disordered
(arsenic-deficient) LaO_0.9F_0.1FeAs_(1-delta) in a wide temperature and
magnetic field range up to 47 T. Because of the large linear slope of Bc2 about
-5.4 T/K to -6.6 T/K near Tc = 28.5 K the T-dependence of the in-plane Bc2(T)
shows a flattening near 23 K above 30 T which points to Pauli-limited behavior
with Bc2(0) about 63-68 T. Our results are discussed in terms of disorder
effects within conventional and unconventional superconducting pairings.Comment: Change of the title as suggested by the Editors, one author added,
typos corrected, references updated, final published versio
Nernst effect of iron pnictide and cuprate superconductors: signatures of spin density wave and stripe order
The Nernst effect has recently proven a sensitive probe for detecting unusual
normal state properties of unconventional superconductors. In particular, it
may sensitively detect Fermi surface reconstructions which are connected to a
charge or spin density wave (SDW) ordered state, and even fluctuating forms of
such a state. Here we summarize recent results for the Nernst effect of the
iron pnictide superconductor , whose ground state evolves
upon doping from an itinerant SDW to a superconducting state, and the cuprate
superconductor which exhibits static stripe
order as a ground state competing with the superconductivity. In , the SDW order leads to a huge Nernst response, which allows
to detect even fluctuating SDW precursors at superconducting doping levels
where long range SDW order is suppressed. This is in contrast to the impact of
stripe order on the normal state Nernst effect in . Here, though signatures of the stripe order are
detectable in the temperature dependence of the Nernst coefficient, its overall
temperature dependence is very similar to that of ,
where stripe order is absent. The anomalies which are induced by the stripe
order are very subtle and the enhancement of the Nernst response due to static
stripe order in as compared to that of the
pseudogap phase in , if any, is very small.Comment: To appear in: 'Properties and applications of thermoelectric
materials - II', V. Zlatic and A. Hewson, editors, Proceedings of NATO
Advanced Research Workshop, Hvar, Croatia, September 19 -25, 2011, NATO
Science for Peace and Security Series B: Physics and Biophysics, (Springer
Science+Business Media B.V. 2012
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